As one flat panel, a liquid crystal display device (liquid crystal display, hereinafter also referred to as an LCD in some cases) has been known. A liquid crystal display device is widely used in a monitor of a personal computer and a portable information terminal device etc., making use of its advantages such as low power consumption, small size, and light weight. In recent years, a liquid crystal display device is also widely used in application to a television.
As a liquid crystal controlling method of LCDs, a twisted nematic (TN) mode has been known. Further, a horizontal electric field mode as typified by an in-plane switching mode and a fringe field switching (FFS) mode has been known. According to a horizontal electric field mode, a wide viewing angle and high contrast can be obtained.
An in-plane switching mode is a display mode in which display is implemented by applying a horizontal electric field to liquid crystals provided between opposing substrates. In an in-plane switching, a pixel electrode and a common electrode to which a horizontal electric field is applied are provided in the same layer. For this reason, liquid crystal molecules located right above a pixel electrode cannot fully be driven, resulting in reducing transmittance.
On the other hand, in an FFS mode, a common electrode and a pixel electrode are arranged with intermediation of an interlayer insulating film, and an oblique electric field (fringe electric field) is thereby generated. For this reason, a horizontal electric field can also be applied to liquid crystal molecules located right above a pixel electrode, and thus liquid crystal molecules located right above a pixel electrode can be fully driven. Therefore, transmittance higher than that of an in-plane switching mode, as well as a wide viewing angle, can be obtained.
In an FFS mode, a fringe electric field is generated between a slit electrode for liquid crystal control provided on an upper layer and a pixel electrode provided on a lower layer of the slit electrode for liquid crystal control with intermediation of the interlayer insulating film. The pixel electrode and the slit electrode for liquid crystal control are formed of a transparent conductive film in this structure, and thus a pixel aperture ratio can be prevented from being reduced. A transparent conductive film is formed of an oxide-based material such as indium tin oxide (ITO) containing indium oxide and tin oxide and InZnO containing indium oxide and zinc oxide.
Further, in an FFS mode, a pixel electrode and a slit electrode for liquid crystal control form a storage capacitor. For this reason, unlike a TN mode, a pattern for a storage capacitor need not necessarily be formed separately in a pixel. This also contributes to a high pixel aperture ratio.
As a switching device of an LCD, a thin film transistor (hereinafter also referred to as a TFT in some cases) has been used. Note that, out of two substrates opposed with intermediation of a liquid crystal layer, a substrate mounted with a TFT is referred to as a TFT substrate. Hitherto, amorphous silicon (a-Si) has been used in a channel layer of a TFT. One reason therefor is that, owing to being amorphous, a film with satisfactory uniformity of characteristics can be formed even on a substrate having a large area. Further, another reason is that, since deposition can be carried out at a comparatively low temperature, an inexpensive glass substrate inferior in heat resistance can be used. Being able to use an inexpensive glass substrate leads to cost reduction, which is desirable in liquid crystal display devices for a general television, in particular.
In recent years, however, a TFT using an oxide semiconductor in its channel layer has been actively developed. An oxide semiconductor has an advantage in that a film of a satisfactorily uniform amorphous state can be stably obtained by optimizing its composition. Further, an oxide semiconductor also has an advantage in that a small-size and high-performance TFT can be realized as the oxide semiconductor has mobility higher than that of a-Si. Accordingly, application of an oxide semiconductor film to a pixel TFT (i.e., a TFT for a pixel) of an FFS mode can realize a TFT substrate having an even higher pixel aperture ratio.
Hitherto, a drive circuit that applies a drive voltage to a pixel TFT is provided in a circuit substrate separate from a TFT substrate, and is externally attached to the TFT substrate. Such a structure is concerned with the demand of a narrower frame in liquid crystal display devices. Specifically, in a case where a TFT of a drive circuit is provided in a frame region (i.e., a peripheral region of a display region) of a TFT substrate and the TFT is formed of a-Si, a comparatively large circuit area for the drive circuit is required in the frame region because mobility of a-Si is low. This leads to increase of a frame region, being unable to satisfy the demand of a narrower frame. For this reason, a drive circuit has been externally attached to a TFT substrate.
However, if a TFT of a drive circuit is formed of an oxide semiconductor having higher mobility, a comparatively small circuit area suffices. For this reason, even when a drive circuit is provided in a frame region of a TFT substrate, the demand of a narrower frame can be satisfied. Further, a drive circuit no longer needs to be externally attached, and thus an LCD can be manufactured at a low cost.
An example of using an oxide semiconductor in a TFT is disclosed in Patent Document 1. According to the disclosure, a second gate insulating film is formed on a first gate insulating film, an oxide semiconductor film is formed on the first and second gate insulating films, and a source electrode and a drain electrode are formed on the oxide semiconductor film. A portion between the source electrode and the drain electrode serves as a channel. The second gate insulating film is formed below a channel in the vicinity of the drain electrode. According to this structure, electrostatic capacity of a channel on the second insulating film is smaller than electrostatic capacity of a channel on the first gate insulating film. With this, the channel in the vicinity of the drain electrode has high resistance, and a TFT having a high withstand voltage can be manufactured.